It's probably related to the nasal consonant
/m/ at the end of the word.
/xānom/ is the formal form of the word, which is spelled
خانُم (though normally without the diacritic).
/xānum/, is the informal form, and is written as
in all of these cases the one with
آ / ā is formal, and the one with
و / u is informal.
but to give you another example:
بادام which in formal speech is pronounced
/bādām/, but in informal speech it's pronounced
/bādām/ (same as formal speech) or
/bādum/ (all three forms), to me none sounds old fashioned, but the one with the
/u/ sounds more informal than the one with the
/o/ (and the one with
/ā/ sounds the most formal, though as i said, it's quite common to pronounce it like this even in informal speech, and in fact it's probably the most common pronounciation in informal speech, out of the three).
also note that this usually doesn't happen with people's names or some other words, for example
بهنام, a given name (and a compound noun:
به / good\better +
نام / name) is not pronounced as
بهنوم in informal language, except by someone who has a specific accent typically associated with roughnecks and thugs especially in lower-class neighborhoods of Tehran; the only place I've ever heard such an accent is in old movies or movies portraying old Tehran, used by roughnecks and thugs; another example from this category is
فرجام / ending\destiny which is NOT a given name, and it's a simple noun (not compound); another example of this category:
سرسام / Meningitis which is a compound noun (
سر / head +
سام / swelling), does not transform to
سرسوم in informal speech..
And with many (Given) names, like
سام not even those old fashioned roughnecks and thugs would spell it with
و / u.
also note that the pluralization suffix
ان doesn't change into
ون in informal speech, since it's never used in informal speech ... in informal speech only the suffix
آ / ā is used for making plurals, which is a shortened version of
درخت ها in formal speech, will be
درختا in informal speech.